Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase. Significant advances in high temperature capabilities have been achieved through the development of nickel and cobalt-base superalloys, and through the use of oxidation-resistant environmental coatings capable of protecting superalloys from oxidation, hot corrosion, etc.
Diffusion aluminide coatings have found wide use as environmental coatings. Diffusion aluminides are generally single-layer oxidation-resistant coatings formed by a diffusion process, such as a pack cementation or vapor (gas) phase deposition, both of which generally entail reacting the surface of a component with an aluminum-containing gas composition. Examples of pack cementation processes are disclosed in U.S. Pat. Nos. 3,415,672 and 3,540,878, assigned to the assignee of the present invention and incorporated herein by reference. In pack cementation processes, the aluminum-containing gas composition is produced by heating a powder mixture of an aluminum-containing donor material, a carrier (activator) such as an ammonium or alkali metal halide, and an inert filler such as calcined alumina. The inert filler is required to prevent powder sintering and promote a uniform distribution of the volatile halide compound around the component, so that a diffusion aluminide coating of uniform thickness is produced. The activator is typically a fluoride or chloride powder, such as NH.sub.4 F, NaF, KF, NH.sub.4 Cl or AlF.sub.3. While pack cementation processes may use the same donor material to aluminize nickel-base and cobalt-base superalloys, a lower amount of donor must be used for nickel-base substrates as compared to cobalt-base substrates.
The ingredients of the powder mixture are mixed and then packed and pressed around the component to be treated, after which the component and powder mixture are typically heated to about 1200-2200.degree. F. (about 650-1200.degree. C.), at which the activator vaporizes and reacts with the donor material to form the volatile aluminum halide, which then reacts at the surface of the component to form the diffusion aluminide coating. The temperature is maintained for a duration sufficient to produce the desired thickness for the aluminide coating.
Aluminum-containing donor materials for vapor phase deposition processes can be an aluminum alloy or an aluminum halide. If the donor is an aluminum halide, a separate activator is not required. The donor material is placed out of contact with the surface to be aluminized. As with pack cementation, vapor phase aluminizing (VPA) is performed at a temperature at which the aluminum halide will react at the surface of the component to form a diffusion aluminide coating.
The rate at which a diffusion aluminide coating develops on a substrate is dependent in part on the substrate material, donor material and activator used. If the same donor and activator are used, nickel-base substrates have been observed to develop a diffusion aluminide coating at a faster rate than cobalt-base substrates. To achieve comparable coating rates, cobalt-based alloys have required higher aluminum activity in the coating chamber, necessitating that different donor materials and/or activators be used. For example, donors with lower aluminum contents (typically chrome-aluminum alloys containing about 30% aluminum by weight) have often been used to coat nickel-base superalloys, while donors with higher aluminum contents (e.g., 45% by weight) have been used for cobalt-base superalloys. Consequently, components formed of a combination of nickel and cobalt superalloys typically have not been aluminized in a single process, but have been required to undergo separate aluminizing steps with the result that considerable additional processing time and costs are incurred.